1. Field of the Invention
The object of the invention is a process for the production of foamed plastics from polyolefins by peroxide crosslinking and foaming with a chemical foaming agent under conditions of normal pressure in the presence of a magnesium compound as an auxiliary foaming agent and, optionally, of other auxiliary agents.
2. Discussion of the Prior Art
It is known to mix polyolefins with organic peroxides and foaming agents. The polyolefins may be mixed with the crosslinking and foaming agents and, optionally, with conventional additives in a roll mill or an extruder, followed by production of the shaped articles, such as plates or continuous sheets, below the temperature of decomposition of the peroxide. The polyolefin then is crosslinked below the decomposition point of the foaming agent by decomposition of the crosslinking agent. The temperature then is raised to the decomposition range of the foaming agent, the polyolefin then being foamed by the gases so generated. The foaming agent preferably used in these so-called pressure-free foaming processes (which are carried out under conditions of normal pressure) is azodicarbonamide. The product obtained is a polyolefin foam having a relatively high density and an irregular foam structure. (U.S. Pat. No. 3,098,831 and German patent application No. 1,694,130.)
In principle, the density of the foamed plastic can be regulated by variation of the amount of foaming agent used. This does not, however, make it possible to obtain at the same time a foamed plastic having a uniform foam structure and surface, especially with low densities of the foamed plastic.
To increase the gas yield, it is known from German Pat. No. 1,936,098 to add to the foaming agent, azodicarbonamide, from 0.05 to 10 weight percent of a chromium compound as an auxiliary foaming agent. However, to obtain an appreciable increase in gas yield, and hence a reduction of the bulk density of the foamed plastic, relatively large amounts of chromium compound are required, and this has proved a drawback in several respects. The foamed plastics undergo discoloration the foam structure in nonuniform, and the surface is irregular. A further drawback is that when such large amounts of chromium compound are used, substantial capital expenditures are necessary for reasons of environmental protection, and these make the process unprofitable.
To lower the decomposition point of the azodicarbonamide, so-called kickers such as zinc, cadmium barium or lead compounds and, as disclosed in German patent application DOS 2,351,515, also magnesium compounds, have been used.
However, in the production of peroxide-crosslinked foamed plastics from a polyolefin by the "pressure-free" foaming method, the use of kickers poses problems. The substances employed interfere with the function of the peroxide, and hence with crosslinking, and in most cases also with the foaming process which follows, as a result of which foamed plastics of excessively high density are obtained. With the decomposition of the foaming agent setting in earlier, there is the danger that crosslinking may not have progressed sufficiently by the time the foaming agent is activated, with the result that foamed plastics having enlarged cells are obtained.
In another foaming process, in which polyethylene is first heated in a closed mold at a molding pressure of 80 bars to the decomposition temperature of the azodicarbonamide and held at that temperature for some time before the foaming process as such begins with the superatmospheric pressure being relieved, the problem of cell enlargement due to kickers is not encountered, or at least not to the same extent as in the "pressure-free" foaming process, the reason being that, as is known, relatively small cells are formed when the thermoplastic melt is saturated with the nitrogen released by the azodicarbonamide foaming agent as the latter breaks down, which in the prior-art processes is brought about by the use of pressure prior to the foaming process as such. (See also Japanese patent application 40577/77.) In these pressure processes, basic magnesium carbonate, magnesium hydroxide and magnesium oxide are used as auxiliary foaming agents in an amount of not less than 5 and up to 100 weight percent, based on the azodicarbonamide used. According to the examples, 25 weight percent of the magnesium compound is needed to obtain a 20 fold enlargement of the shaped article to be foamed. Without auxiliary foaming agent, a 15 fold enlargement is obtained. Such foaming processes which work with superatmospheric pressure therefore cannot be compared with foaming processes in which normal pressure is employed throughout.
The cannot be lumped together with the "pressure-free" foaming processes also for another reason: The superatmospheric-pressure foaming processes lend thermselves only to the batchwise production of shaped articles in molds, whereas processes employing conditions of normal pressure throughout permit the production of continuous foamed-plastic webs or sheets. (See, for example, German patent application DAS 1,694,130.)
Based on the prior art foaming process in which conditions of normal pressure are employed throughout, the invention has as its object to provide a uniform foamed plastic having low bulk density, especially by a "pressure-free" foaming process.